The present invention relates to method and apparatus for flattening the passband of optical Dense Wavelength Division Multiplexer (DWDM) or Demultiplexer devices having rounded passband shapes.
The implementation of a low cost, low insertion loss optical Dense Wavelength Division Multiplexer (DWDM) or Demultiplexer is of great advantage in high capacity optical transmission system. Unfortunately, most high capacity DWDM devices have a high insertion loss or a high cost of manufacture associated with them. A design capable of accomplishing dense wavelength division multiplexing with low insertion loss is obtained by using an unbalanced Mach-Zehnder (UMZ) interferometer. However, UMZ filters suffer from a characteristic problem of a narrow pass bandwidth near the transmission peak. This narrow pass bandwidth near the peak is undesirable because it provides unwanted shaping of the signal. This problem is further exacerbated when a plurality of UMZs are cascaded in order to perform multiple channel demultiplexing or multiplexing.
U.S. Pat. No. 5,809,190 (Chen), issued on Sep. 15, 1998, discloses an unbalanced Mach-Zehnder (UMZ) interferometer capable of accomplishing dense wavelength division (DWD) multiplexing with low insertion loss. More particularly, Chen discloses apparatus and method of making a fused dense wavelength division multiplexer (DWDM) using a fused-biconical taper technique. The DWDM comprises multiple Multi-window Wavelength Division Multiplexers (MWDMs) which are cascaded in several stages where the MWDMs in each stage have an identical window spacing. For an N-channel DWDM, there are a predetermined plurality of DWDMs in each stage, and the stages are cascaded to form the MWDM. Unfortunately, the UMZ is by nature sensitive to temperature fluctuations of the environment, and typical temperature fluctuations expected in the terminal environment can render the DWDM device unusable. Therefore, the disclosed UMZ device is unstable because of the occurrence of variations in phase due to temperature fluctuations. A similar device is also discussed in an article titled xe2x80x9cFused-Coupler Technology for DWDM Applicationsxe2x80x9d by F. Gonthier in the magazine Fiber Optic Product News, September 1998, at pages 54 and 56.
It is desirable to provide an optical filtering arrangement that is especially designed to flatten the passband of a unbalanced Mach-Zehnder filter. It is further desirable to provide an optical filtering arrangement that can be applied to enhance the passband flatness of any optical filter or DWDM arrangement with a rounded type of passband shape.
The present invention is directed to method and apparatus for flattening the passband of optical Dense Wavelength Division Multiplexer (DWDM) or Demultiplexer devices having rounded passband shapes.
Viewed from an apparatus aspect, the present invention is directed to an optical arrangement which comprises an optical filter and an optical flattening filter. The optical filter is responsive to an input signal for generating at an output thereof one of a group consisting of a periodic and a periodic output response waveform having a rounded passband shape. The optical flattening filter is coupled in cascade with the optical filter between an input and the output of the optical arrangement for generating a response waveform having a spectral profile which is complementary to the periodic or aperiodic output response waveform of the optical filter and interacts with the response waveform of the optical filter such that each peak in a resultant response waveform at the output of the optical arrangement is substantially flattened to provide a passband which is wider than the response waveform provided by the optical filter.
Viewed from a process aspect, the present invention is directed to a method of generating a substantially widened flat passband at an output of an optical arrangement. The method comprises a first step of generating, in response to an input signal to an optical filter, one of a group consisting of a periodic output response waveform and a aperiodic output response waveform having a rounded passband shape. The method further comprises a second step of generating a response waveform having a spectral profile which is complementary to the periodic or aperiodic output response waveform of the optical filter in an optical flattening filter coupled in cascade with the optical filter between an input and the output of the optical arrangement. The method further comprises a third step of generating a passband which is wider than the response waveform provided by the optical filter at the output of the optical arrangement by the interaction of the response waveforms of the optical filter obtained in the first step and the optical flattening filter obtained in the second step such that each peak in a resultant response waveform at the output of the optical arrangement is substantially flattened.
The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.